The present invention relates to an electric motor, comprising an electromagnetically actuated mechanical brake and to a method of operating an electric motor with an electromagnetically actuated mechanical brake.
Electric machines can be controlled by their rotational speed. Electronic means are now available with which such machines can be powered via converters by means of either alternating or direct current supplies. A known method of improving the controllability of such machines during braking, as is required in many applications of such machines, is to operate the machine as a generator and to convert the energy so produced during braking into heat by way of a load resistance. However, the provision of such an additional load resistance involves an increase in expense and complexity of construction of the machine, and furthermore the heat produced must be dissipated by means of additional extra apparatus.
It is further known that such machines, when operated predominantly as electric motors, can be provided with mechanical brakes that can be released or raised by an electromagnet arrangement. Thus, when current is supplied to the electric motor, it is also supplied to the excitation coil of an electromagnetically actuated mechanical brake, and when no current is supplied to the motor, the brake operates to immobilize the motor.
The object of the present invention is to provide an electric motor and a method of operating an electric motor wherein the braking process is substantially simplified with respect to the prior art.
According to a first aspect of the present invention there is provided a method of operating a converter-controlled electric motor with an electromagnetically actuated brake comprising an excitation coil wherein the method comprises the step of supplying electrical energy to the excitation coil of the brake during braking in a generational mode of the motor for at least one of storage temporarily as magnetic energy and conversion to thermal energy.
According to a second aspect of the present invention there is provided an electric motor comprising an electromagnetically actuated mechanical brake; an excitation coil forming part of the brake; a converter; and a brake control means connected to the electric motor and the excitation coil in such a way that during braking of the electric motor in a generational mode of operation electrical energy produced by the motor is supplied to the excitation coil for at least one of temporary storage and conversion into heat.
By virtue of the invention, the electrical energy produced while the equipment is operating in a generational mode during braking is converted into heat in an excitation coil of the electromagnetically releasable mechanical brake. Conventionally, during mechanical braking the excitation coil is not supplied with any energy. However, the invention makes it possible to operate without a braking resistance and furthermore to exploit the inductivity function of the coilxe2x80x94that is, its particular dynamic action, which the coil exhibits in contrast to an additional ohmic resistance such as is customarily employed.
Preferably, when the brake is applied strongly during operation in a generational mode, a current is supplied to the excitation coil that is considerably larger than that used to release the brake or keep it raised in an off position. That is, advantage is taken of electrical properties of the coil that in normal operation, for the usual release of the brake, are not exploited. However, the excitation coil can be supplied for a certain period with a much larger current than is supplied during normal operation to release the brake or to keep it in an off position.
Preferably, the thermal load on the excitation coil is ascertained and the current supplied to the coil is kept below a predetermined value beyond which the thermal load would exceed a predetermined temperature. It can thereby be ensured that no damage is caused by overheating. Preferably when there is a risk of thermal overloading, the current is limited to the maintenance level for the excitation coil, i.e. the amount of current that flows through the electric motor during maintained operation of the electric motor and is tailored to the excitation coil.
In a preferred embodiment of the invention the thermal load is ascertained by measuring the temperature of the excitation coil. With this kind of load measurement particularly accurate results can be expected. In an alternative embodiment of the invention, which can also be employed as an adjunct to the first embodiment, the current and/or voltage supplied to the excitation coil are/is monitored and these values, in combination with parameters specific to the excitation coil, in particular the thermal time constant of the excitation coil, are processed in such a way that not only is the momentary thermal load known but also, at any time, it can be estimated how long the excitation coil can continue to be operated with the present braking performance before the excitation current must be reduced. By this means the braking behavior can be optimized.
Preferably in addition to the thermal load on the excitation coil, the ambient temperature is also measured. As a result, the excitation coil is still more reliably protected from overloading. The same applies to a measurement of the temperature at the electric motor by means of a corresponding temperature sensor, which is usually present in any case. Once the brake has been mounted on the electric motor, along with its excitation coil, and heat flow is occurring, the temperature of the electric motor also provides a measure of the amount of heat that can still be conducted to the excitation coil.
The various aspects of the present invention will now be described by way of example with reference to the accompanying drawings.